Methods and apparatus for controlling a fluid damper

ABSTRACT

A spring for a suspension is described. The spring includes: a spring chamber divided into at least a primary portion and a secondary portion, and a fluid flow path coupled with and between the primary portion and the secondary portion. The fluid flow path includes a bypass mechanism, wherein the bypass mechanism is configured for automatically providing resistance within the fluid flow path in response to a compressed condition of the suspension.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and benefit of co-pending U.S. patent application Ser. No. 15/394,622, filed on Dec. 29, 2016, entitled “METHODS AND APPARATUS FOR CONTROLLING A FLUID DAMPER” by John Marking, having Attorney Docket No. FOX-0039US.CON2, which is herein incorporated by reference, and assigned to the assignee of the present application.

The application Ser. No. 15/394,622 claims priority to and benefit of U.S. patent application Ser. No. 14/590,836, now U.S. Pat. No. 9,550,405, filed on Jan. 6, 2015, entitled “METHODS AND APPARATUS FOR CONTROLLING A FLUID DAMPER” by John Marking, having Attorney Docket No. FOX-0039US.CON, which is herein incorporated by reference, and assigned to the assignee of the present application.

The application Ser. No. 14/590,836 claims priority to and benefit of U.S. patent application Ser. No. 12/902,239, now U.S. Pat. No. 8,955,653, filed on Oct. 12, 2010, entitled “METHODS AND APPARATUS FOR CONTROLLING A FLUID DAMPER” by John Marking, having Attorney Docket No. FOXF/0039US, which is herein incorporated by reference, and assigned to the assignee of the present application.

The U.S. patent application Ser. No.12/902,239 claims priority to and benefit of U.S. provisional patent application Ser. No. 61/250,927, filed Oct. 13, 2009, entitled “SYSTEM FOR MAGNETIC-RHEOLOGICAL (MR) FLUID”, by John Marking having Attorney Docket No. FOXF/0039L, which is herein incorporated by reference, and assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to shock absorbers for vehicles. More particularly, the disclosure relates to fluid dampers. More particularly still, the disclosure relates to methods and apparatus for controlling and managing dampening through the selective use of dampening fluid having variable flow characteristics.

Description of Related Art

Magneto rheological fluid (MR fluid) is a variable character fluid comprising a (e.g. colloid like) suspension of micrometer-sized particles in a carrier fluid, often a type of oil. When subjected to a magnetic field, the fluid greatly increases its apparent viscosity and/or shear strength. The particles, which are typically micrometer or nanometer scale spheres or ellipsoids, are active when subjected to a magnetic field (e.g. such as iron particles) and are distributed randomly and in suspension within the carrier liquid under normal circumstances. When a magnetic field is applied to the liquid suspension, however, the particles (usually in the 0.1-10 μm range) align themselves along lines of magnetic flux. When the fluid is contained between two poles (typically of separation 0.5-2 mm), the resulting chains of particles restrict the movement of the fluid, perpendicular to the direction of flux, effectively increasing its viscosity and/or shear strength. The yield stress of the fluid when it is “activated” or in an “on” magnetized state can be controlled very accurately and quickly (typically a few milliseconds) by varying the magnetic field intensity.

There are problems arising from the use of variable viscosity fluids, like MR fluid in mechanical applications. For example, even in the absence of electromagnetic energy, MR fluid is very dense, resulting in much greater viscosity and strength (up to four times) compared to “normal” fluids. Mechanical systems using such fluids may not be capable of handling corresponding dynamic loads.

Another problem with the MR fluid is its abrasiveness. This abrasiveness is caused by the ferrous particles suspended in the oil as they can have a sandpaper effect on all of the moving parts. Mechanical systems employing such fluids may be rapidly worn out.

What is needed is a damper for a suspension system that utilizes variable rheology fluid in a manner that avoids problems associated with use of such fluid.

SUMMARY OF THE INVENTION

The present invention generally includes a fluid damper comprising a first fluid-filled chamber, a second chamber filled with a fluid having variable flow characteristics and at least partially displaceable by the first fluid, and a gas chamber, the gas chamber compressible due to the displacement of the second chamber. In one embodiment, the fluid in the second chamber is a variable rheology fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features can be understood in detail, a more particular description, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of the invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG. 1 is a section view of a damper according to one embodiment.

FIG. 2 is a section view of the damper of FIG. 1 with the damper in a compression stroke.

FIG. 3 is a schematic diagram showing one control arrangement for dampers.

DETAILED DESCRIPTION

FIG. 1 is a section view of a damper 100. The damper includes a housing 105 as well as a piston 110 and rod 115 for reciprocating within the housing in compression and retraction or “rebound” strokes. The housing includes a first chamber 120 filled with a first fluid 121, and the chamber is dividable into a compression side 120 a (shown in FIG. 1) and a rebound side 120 b (FIG. 2). The piston 110 is often provided with fluid pathways therethrough including shims 125 a, b which permit fluid to pass between sides 120 a, 120 b of the first chamber 120 while providing predetermined damping flow resistance. For example, during a compression stroke shims 125 a are displaceable to permit fluid to move through the piston in an upwards direction. Similarly, during a rebound stroke, shims 125 b permit fluid to flow back into the compression side 120 a of the first chamber 120. In addition to shims 125 a, fluid metering in a compression stroke is controlled by a valve assembly consisting of an axially adjustable member 130 a which permits and restricts fluid flow. Member 130 a is adjustable by a user through the manipulation of a knob 130 b having a detent mechanism 130 c to indicate the axial position of member 130 a. The valve assembly is adjustable to permit fluid from the compression side 120 a of the chamber to flow to the rebound side 120 b through a fluid path 130 d. Rebound flow may also flow, in an opposing direction during rebound, through that path.

An outer surface of the rod 115 is sealed and centered relative to an inner surface of the housing 105 with a seal and rod bushing 135, and an outer surface of the piston is sealed with an inner surface of the housing by another seal 140. A wiper seal 145 prevents contamination from entering the housing 105 and bumpers 150 a, 150 b prevent the piston 110 from bottoming or topping out as it reciprocates in the housing 105. At an upper end of the rod 115, a mounting eye 155 a permits the rod to be mounted to another part of the vehicle suspension system (not shown), and another mounting eye 155 b at a lower end of the housing permits the housing portion of the damper 100 to be mounted to a vehicle frame.

In operation, the damper 100 of FIG. 1 works in conjunction with a spring member (not shown). In one embodiment comprising a mechanical spring (not shown) the damper includes a first spring flange 160 a mounted on the rod and a second spring flange 160 b mounted on the housing where a spring would be situated axially between the two spring flanges. In one embodiment a coil spring (not shown) extends between the first and second spring flange to provide resistance to compressive forces during operation of the damper 100. Threads 160 c formed on an outer surface of the housing 105 permit adjustment of spring flange 160 b in order to adjust the preload of a coil spring. In one embodiment a damper hereof is used in conjunction with an air spring. Some exemplary air spring configurations are shown in U.S. Pat. No. 6,135,434 (“'434 Patent) which patent is entirely incorporated herein by reference. Referring to FIGS. 3, 4, 5 of the '434 Patent, the chamber 200, fluid 201 barriers 210 and magnet 250 would be situated within damping chamber 21A in a fashion generally as described herein.

Also included within the damper housing 105 is a second chamber 200 formed adjacent the compression side 120 a of the first chamber 120 and fluidically isolated from the first chamber. The second chamber includes a fluid having variable flow characteristics like a variable rheology (e.g. viscosity, shear strength) fluid 201 which, in a present embodiment is an MR fluid. A fill valve 203 permits the fluid 201 to be inserted into the chamber 200. In one embodiment, the fluid 201 comprises particles 202 having magnetic properties as illustrated in the chamber. The chamber 200 is housed between two flexible end walls 210 a, b at a first and second end of the chamber. In one example, the end walls are made of an elastomer-type material which is affixed at an outer perimeter to the inside of the housing wall. In one embodiment each end wall is pre-fabricated with (e.g. bonded to) its own perimeter ring. The rings (having seals about an outer diameter thereof for sealing engagement with an inner diameter of the housing 105) are then installed in an interior of the damper housing and retained in that position by a shoulder member, or snap ring, or suitable axial retainer or combination thereof (not shown). The end walls 210 a, b are constructed and arranged to be flexible (and optionally highly elastic) so that portions of the walls are displaceable longitudinally within the housing 105 and each can be displaced to accommodate axial movement of the volume of fluid 201. In this manner, the fluid 201 in the second chamber 200 is displaceable longitudinally within the damper housing 105 depending upon pressures and forces acting within the damper as will be further explained in relation to FIG. 2.

In addition to end walls 210 made of an elastomer material, there are a number of other constructions that could serve a similar purpose. In one example, the end walls are made of a metallic or non-metallic material that in a normal state, includes corrugations or folds (e.g. bellows). When acted upon by pressurized fluid or gas however, the corrugations at least partially straighten out, permitting the walls some flexibility to move the fluid in relation to pressures within the housing 105. Thereafter, the walls return to a somewhat folded shape. In another instance, the second chamber is simply formed between a pair of floating pistons spaced far enough apart to house the fluid volume.

Also disposed within the second chamber 200 is a magnet 250. In one embodiment the magnet 250 is a permanent magnet of a pre-selected strength for providing a desired effect on the fluid 201. In one embodiment the magnet 250 is a “doughnut” shaped magnet. In one embodiment, providing for variable magnetic field (and flux), the magnet 250 comprises an electromagnet. As described herein, electromagnets use electric current to generate a magnetic field which can be turned “on” or “off” or may be modulated to higher or lower flux states as desired. When connected to a DC voltage or current source, the electromagnet becomes energized and creates a magnetic field like a permanent magnet. Electromagnets are often made from relatively soft yet conductive metal winding which quickly dissipates the induced magnetism after the current is switched off. In one embodiment, the electromagnet 250 is doughnut-shaped and forms an orifice 255 at its center which is constructed and arranged to meter the fluid in the second chamber 200 as the fluid is forced from one side of the magnet to the other during compression and rebound strokes respectively. When electric current is supplied to the electromagnet, magnetic flux lines are imposed within the fluid 201 and magnetic particles 202 in the fluid 200 align with such flux lines and become relatively stationary with respect to the magnet. This effect increases the resistance to movement of the fluid in the area of the orifice 255. The magnetic flux density is proportional to the magnitude of the current flowing in the wire of the electromagnet. As such the higher the current that is supplied to the magnet the more resistant to flow will be the fluid 201 because more fluid, further into the center of the orifice will be subjected to the magnetic field thereby “freezing” a greater thickness of particles proximate the orifice.

In order to lessen the abrasive effects of the MR fluid, the magnetic orifice 255 may be coated with an abrasion resistant substance. In one instance, the magnet can be coated with a super hard yet tough material such as for example tungsten carbide with relatively medium to high nickel and/or cobalt content. In another embodiment, the magnet can be coated with medium hard rubber such as Nitrile shore A 70 or 80(or other elastomer). In one embodiment the magnet can be coated with ceramic or super hard material which in turn is coated with rubber. In one embodiment the orifice may be coated with any suitable combination of hard or compliant abrasion resistant materials. Further the magnet may be surrounded by a fluid retaining barrier to ensure that the magnet is isolated from the fluid and the retaining barrier may in turn be coated for abrasion resistance in any suitable manner. In each of these examples, the material of the magnet is protected from abrasion, fluid invasion and/or corrosion while the coating is chosen to minimize its effect on the magnetic properties of the component (e.g. the coatings and barriers are preferably materials lacking in magnetic properties such as, for example, 300 series stainless steels, noble metals and alloys or polymers or ceramics).

While the embodiment shown includes an electromagnet 250 in the interior of the damper housing 105, the magnet could be annularly arranged on an exterior of the housing and still effect the MR fluid in a way that increases its flow resistance. Electromagnet arrangements external to a damper are disclosed in U.S. Pat. No. 7,422,092 and that patent is incorporated by reference herein in its entirety. In one embodiment a magnet (functionally 250) may be circumferentially intermittent so that is within the housing or without the housing or a combination thereof. In one embodiment a series of magnet are placed axially adjacent the fluid 201 so that various magnets may be activated in series at various points in the stroke of the damper to result in a position dependent damping characteristic.

In one embodiment a gas chamber 300 which is filled with nitrogen to some predetermined pressure is in pressure communication with the second chamber 200. A fill valve 301 permits pressurization of the gas chamber. The purpose of the gas chamber 300 is to act as a compressible reservoir whereby fluid volume from the first 120 chamber can displace a portion of the gas chamber as the piston rod 115 (and its associated volume) moves into the damper housing 105. Additionally, the gas chamber provides a non-linear, spring-like resistance during a compression stroke of a damper due to its pressure acting on an end area of rod 115. In one embodiment the chamber 200 including fluid 201, barriers 210 and magnet 250 are placed between a compression chamber and a reservoir gas charge in place of, for example, intensifier assembly 780 of FIG. 32 of U.S. Pat. No. 7,374,028 (“'028 Patent”) which patent is entirely incorporated herein by reference. In one embodiment, the chamber 200 and fluid 201 with barriers 210 and magnet 250 are placed in parallel with an intensifier assembly like, for example, intensifier 780 of the '028 Patent. In one embodiment, the chamber 200 and fluid 201 with barriers 210 and magnet 250 are placed in series with an intensifier assembly like, for example, intensifier 780 of the '028 Patent. While the embodiment shown includes a gas chamber, the compressible portion of the damper could be a mechanical spring disposed, for example in an atmospheric chamber.

FIG. 2 is a section view of the damper 100 of FIG. 1 illustrating the damper during a compression stroke. As illustrated by the arrows 270, the piston and rod are moving into and towards a lower end of the first chamber 120 and the first fluid 121, which in the embodiment of the Figures is a relatively “Newtonian” fluid, is being metered through shims 125 a in the piston from a compression 120 a to a rebound 120 b side of the first chamber 120. Also illustrated in FIG. 2, the second chamber 200 with its flexible end walls 210 a, b is being displaced downwardly and in turn, is compressing the gas chamber in order to compensate for a reduction in volume in the housing 105 due to the volume of the piston rod 115 as it enters the housing 105.

In FIG. 2, the second chamber 200 and fluid 201 therein are also being displaced relative to the electromagnet 250 with at least part of the MR fluid in the second chamber 200 having been urged, during compression stroke, through the orifice 255 formed in the center of the electromagnet 250. In FIG. 2, the electromagnet 250 is illustrated in an “on” condition where electric current is being provided to the magnet. As illustrated, particles 202 in the MR fluid have gathered (and “bunched”) in the orifice due to the magnetic field generated by the magnet. The effective result is an increased flow resistance of the MR fluid in the area of the orifice 255 and correspondingly increased dampening in the compression stroke of the damper since the orifice (through which the MR fluid must pass as the second chamber 200 is displaced) has effectively been made smaller by the clustered particles 202. It is noteworthy that the same mechanism can be selectively activated, or deactivated, during rebound to achieve a desire rebound damping resistance as the fluid 201 flows back “up” through the orifice. As mentioned herein, the magnetic flux density brought about is proportional to the magnitude of the current flowing in the wire of the electromagnet. In other words the electromagnet controlled orifice, in combination with a magnetically sensitive fluid, can operate as a valve with an infinite number of settings (including fully “open”, fully “closed” and all points between) depending on the applied current.

While the gas chamber 300 is shown housed in the main damper housing 105, the gas chamber could be remotely located in a separate housing and the second chamber with the MR fluid could also be disposed in the separate housing with fluid communication between the main and remote housings (for example refer to the '028 Patent). In the example of a remote gas chamber, the communication path between the two housings would still permit the second chamber 200 to be displaced, thereby moving the variable viscosity MR fluid relative to the electromagnet 250. Remote gas chambers/reservoirs are shown and described in US patent application no. 2010/0170760 assigned to the owner of the present patent application and that co-pending application is incorporated herein by reference in its entirety.

The damper 100 disclosed herein is intended for use in vehicles, including bicycles and any other type suspended vehicle or motor vehicle. When used with bicycles for example, the electromagnet 250 can be battery powered using power from an existing battery (such as for a head light, for example) or used with its own on-board battery. In one embodiment the magnet could be a permanent magnet and could be mechanically moved closer or further from the fluid 201 and/or orifice to facilitate a greater or lesser magnetic effect. In one embodiment a magnetic field “insulator” (such as for example a shunted conductive sheath or merely a non-conductive spacer) could be selectively interposed between the magnet and the fluid 201/orifice for creating a stronger or lesser magnetic field within the fluid 201 (e.g. proximate the orifice). Permanent magnet embodiments may not require any external power source where manual manipulation may be used to perform the function of moving either the magnet or an insulator or any suitable combination thereof to alter the strength of the magnetic fields within the fluid 201.

An electromagnet's strength is determined by the material in the core; the amount of current in the wire; and the number of turns that the wire makes around the core. Therefore, depending upon the physical characteristics of the magnet, a single AA battery can power the electromagnet disclosed herein. When used with a motor vehicle, the magnet and any control components related to it can easily be powered by the vehicle's battery or alternator (i.e. onboard electrical generation).

While the electromagnet is shown in its “on” position in the compression stroke of FIG. 2, the magnet might be in an “off” position during a rebound stroke when the piston and rod are returning to an upper end of the housing. The magnet's ability to become quickly de-energized once electric current is removed permits its use during one stroke, or a part of one stroke and not the opposite stroke. Such attribute greatly enhances the selectivity which can be applied to the use of the magnetic flow control function. The “on” and “off” conditions are controlled by a switch or potentiometer which can be manually operated or can be automatically operated (e.g. with a microprocessor and solenoids if needed) based upon one or more sensed conditions within the shock absorber or operational conditions of the vehicle.

When used with a motor vehicle, especially an automobile, each wheel of the vehicle can be equipped with a damper 100 having an MR fluid-filled chamber. In these instances, a control system can permit the dampers to work in unison or separately depending upon terrain conditions and how a logic/control unit is programmed. FIG. 3 shows a schematic diagram of a remote control system 500 based upon any or all of vehicle speed, damper rod speed, and damper rod position. In one embodiment, the system is designed to automatically increase dampening in a shock absorber in the event a damper rod reaches a certain velocity in its travel towards, for example, the bottom end of a damper at a predetermined speed of the vehicle. In one embodiment the system adds dampening (and control) in the event of rapid operation (e.g. high rod velocity) of the damper to avoid a bottoming out of the damper rod as well as a loss of control that can accompany rapid compression of a shock absorber with a relative long amount of travel. In one embodiment, the system adds dampening (e.g. orders the magnet to its “on” position) in the event that the rod velocity in compression is relatively low, but the rod progresses past a certain point in the travel. Such configuration aids in stabilizing the vehicle against excessive low rate suspension movement events such as cornering roll, braking and acceleration yaw and pitch and “g-out.”

FIG. 3 illustrates, for example, a system including three variables: rod speed, rod position and vehicle speed. Any or all of the variables shown may be considered by logic control unit 502 in controlling the electromagnet 250. Any other suitable vehicle operation variable may be used in addition to or in lieu of the variables 515, 505, 510 such as for example piston rod compression strain, steering wheel position, brake pedal position, accelerator pedal position, eyelet strain, vehicle mounted accelerometer data or any other suitable vehicle or component performance data or combination thereof. In one embodiment, a suitable proximity sensor or linear coil transducer or other electro-magnetic transducer is incorporated in the dampening cylinder to provide a sensor to monitor the position and/or speed of the piston (and suitable magnetic tag) with respect to the cylinder. In one embodiment, the magnetic transducer includes a waveguide and a magnet that is joined to the cylinder and oriented such that the magnetic field generated by the magnet passes through the piston rod and the waveguide. Electric pulses are applied to the waveguide from a pulse generator that provides a stream of electric pulses, each of which is also provided to a signal processing circuit for timing purposes. When the electric pulse is applied to the waveguide a magnetic field is formed surrounding the waveguide. Interaction of this field with the magnetic field from the magnet causes a torsional strain wave pulse to be launched in the waveguide in both directions away from the magnet. A coil assembly and sensing tape is joined to the waveguide. The strain wave causes a dynamic effect in the permeability of the sensing tape which is biased with a permanent magnetic field by the magnet. The dynamic effect in the magnetic field of the coil assembly due to the strain wave pulse, results in an output signal from the coil assembly that is provided to the signal processing circuit along signal lines.

By comparing the time of application of a particular electric pulse and a time of return of a sonic torsional strain wave pulse back along the waveguide, the signal processing circuit can calculate a distance of the magnet from the coil assembly or the relative velocity between the waveguide and the magnet. The signal processing circuit provides an output signal, digital or analog, proportional to the calculated distance and/or velocity. Such a transducer-operated arrangement for measuring rod speed and velocity is described in U.S. Pat. No. 5,952,823 and that patent is incorporated by reference herein in its entirety.

While a transducer assembly located at the damper measures rod speed and location, a separate wheel speed transducer for sensing the rotational speed of a wheel about an axle includes housing fixed to the axle and containing therein, for example, two permanent magnets. In one embodiment the magnets are arranged such that an elongated pole piece commonly abuts first surfaces of each of the magnets, such surfaces being of like polarity. Two inductive coils having flux-conductive cores axially passing therethrough abut each of the magnets on second surfaces thereof, the second surfaces of the magnets again being of like polarity with respect to each other and of opposite polarity with respect to the first surfaces. Wheel speed transducers are described in U.S. Pat. No. 3,986,118, which is incorporated herein by reference in its entirety.

In one embodiment, as illustrated in FIG. 3, a logic unit 502 with user-definable settings receives inputs from the rod speed 510 and location 505 transducers as well as the wheel speed transducer 515. The logic unit is user-programmable and depending on the needs of the operator, the unit records the variables and then if certain criteria are met, the logic circuit sends its own signal to the magnet to either turn “on” or “off”. Thereafter, the condition of the electromagnet 250 is relayed back to the logic unit 502.

While the examples herein refer to the electromagnet 250 as being in an “on” or “off” position, it will be understood that the nature of the electromagnet permits it to be energized to an infinite number of positions between “off” and fully “on”. For example, a logic control unit 502 can be programmed to energize the magnet 250 to some intermediate level based upon a corresponding level of input from a sensor. These incremental adjustments of energy (and the resulting incremental adjustments to dampening) are fully within the scope of the invention.

As the forgoing illustrates the invention addresses problems associated with using variable rheology fluids in mechanical systems. The second chamber serves to keep the MR fluid close to the electromagnet and reduces the amount of MR fluid necessary for use in the damper while isolating the fluid from the piston, seals and other parts of the damper that may be sensitive to wear and damage from the abrasive particles contained in the MR fluid.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. 

What we claim is:
 1. A reservoir-type fluid damper comprising: a damper body having a longitudinal axis and at least partially defining a fluid chamber containing damping fluid; a piston rod, a portion of said piston rod movable along the longitudinal axis of, and within, said damper body, wherein during a compression stroke, said portion of said piston rod moves further into said damper body; a pressure-sensitive compression control valve assembly comprising: a) a partition separating said fluid chamber into a compression chamber and a reservoir chamber, said partition having at least one compression flow port, wherein during said compression stroke, said piston rod expels said damping fluid from said compression chamber to said reservoir chamber through said at least one compression flow port; b) an intensifier piston, said intensifier piston having a first cross-sectional area and a second cross-sectional area, said second cross-sectional area being larger than said first cross-sectional area, at least one movable element associated with said reservoir-type fluid damper, said at least one movable element having: a) a first portion having a compression force of at least one spring assembly transmitted thereto; and b) a second portion contacting said damping fluid in said reservoir chamber, whereby an amount of force transmitted to said first portion of said movable element by said spring assembly is at least partially converted to a fluid pressure in said reservoir chamber by said second portion of said movable element; whereby resistance to fluid flow through said pressure-sensitive compression control valve assembly is at least partially determined by fluid pressure in said reservoir chamber; a second chamber disposed adjacent said pressure-sensitive compression control valve assembly and between said damper body and said reservoir chamber, said second chamber having a variable rheology fluid disposed therein, said second chamber at least partially displaceable by said damping fluid moving between said compression chamber and said reservoir chamber, said second chamber fluidically isolated from said damping fluid and wherein movement of said second chamber causes a volume change in a compressible chamber disposed within said reservoir chamber; and a magnet disposed adjacent to said second chamber, said magnet configured to provide a magnetic field which affects said variable rheology fluid. 